# Telescope light gathering power and resolution

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I'm a little confused. Could you help me?

The question is; If we increase the radius of a telescope twice, how much does the light gathering power change? What about resolution?

Is it increase 2^2? For second part i know As the diameter of the telescope's objective increases, the resolving power increases.

Thanks

The collecting area of a telescope is roughly proportional to the square of its radius (for a circular aperture). I say roughly because you also have to factor in the small fraction of light blocked by the secondary if we are talking about reflecting telescopes.

In principle the angular resolution decreases as the reciprocal of the radius (that is, the smallest resolvable angle gets smaller). In practice, this may not be the case for ground-based telescopes without adaptive optics capabilities, where the smallest resolvable angle could be determined (for large telescopes at least) by turbulence in the atmosphere (a.k.a. the seeing).

## Telescope common aperture size comparison chart

Since there is usually a lot of talk about telescope aperture sizes and how much light gathering power they have, I decided to do a proper graphic, that contains basic info on most common aperture sizes and their light gathering area. For refactors and the small-medium reflectors. I also removed the secondary mirror obstruction areas, so the mirrors have provided the actual light gathering area, calculated from the exposed surface only (subtracted secondary obstruction). For the dimensions of the reflectors, I took the Synta mirror specs, with their smallest corresponding secondary mirrors. The 6", 8" and 10" inch mirrors have the same dimensions of the secondary mirror across different brands, but there were some minor secondary mirror size differences between 12" and 14" reflectors, between Orion and SkyWatcher.

The shapes are precise and drawn with CAD tools. I made this to as a reference point for beginners and experienced scope buyers, to get an idea what the light gathering power increases are from scope to scope, from aperture to aperture.

The full resolution graphic is too big for direct posting, and so I decided to make a permanent home for it on the link below: (since it will otherwise be removed after some time from the free upload/share sites)

I learned a lot on these forums, so here and there I will try to give something back if I can, from my big bag of different skills.

Edited by nicknacknock, 12 January 2019 - 10:05 AM.

### #2 sg6

Is it me or are the "actual light gathering areas" incorrect?

A 14", 350mm dia, reflector having a light gathering area of 90.930mm 2

Should it read 90,930 as in"," not "." ?

### #3 pregulla

Some countries use comma as decimal separator.

### #4 Recretos

Well, in the metric system its usual to separate thousands by dots and decimals by commas.
The imperial system is of course opposite. We can never seem to get along.

It should be seen what is the decimal separator when you look at the main reference point, the human eye, which has a light gathering area of 38,5mm2.

### #5 SeattleScott

Good illustration. Naturally it ignores the fact that lenses have higher light transmission than mirrors, but it is hard to account for those differences due to variability in coatings and such.

### #6 Recretos

Yes that is true, so i decided to stick with the area. Most online telescope stores give the light gathering area by the full aperture, not compensating for the central obstruction.

There are many different coatings for lenses and mirrors, so it is impossible to account for that on a general basis. That is usually done on a scope-to-scope level.

For example, Synta mirrors are said to have a range of 88-91% reflectivity (numbers seem to vary by sources), while some like the GSO, are selling deluxe dobs with supposed 94% reflectivity on both mirrors.

Comparing a 10" 94% mirror to a 12" 89%, and considering that secondary mirrors are usually the same, it gives us a system reflectivity of 88,4% for the 10" and 79,2% for the 12".

If we apply that to the light gathering area (LGA x %reflectivity) we get 41.057mm2 full-light gathering area for the 10" mirror. Now this number doesn't mean that is the actual area that gathers light, but its just a calculated number that shows how much of the effective light gathering area is "theoretically" lost due to not perfect reflectivity on both mirrors. This calculated number would be a better estimate to compare to refractors for example, in which you have to compensate for transmittance if you want accurate numbers.

The 12" mirror with total 79,2% system reflectivity has a full-light gathering area of 52.943mm2. So compared to the 10" mirror in this example, its now only a 29% effective light gathering area increase, compared to the previous 44% for the raw area size difference. That is a 15% effective reduction when compensating for the coatings. This is just a practical example how big of a role the mirror coatings can play.

On the refractor side, in the present days even the cheapest Chinese achromatic lenses have light transmittance over 95%, with the commercial grade products from major brands around 97% or better.

All this is of course for the light that gets to the focal plane (to the eyepiece). In refractors we have to also account for the star diagonal reflectivity/transmittance if we want to make it perfect.

Then the view also heavily relies on the quality of the eyepiece. This can usually have a bigger impact on the image, rather than a few % better/worse coatings.

Basically the difference from scope to scope can be very big even in the same aperture range, so its impossible to make a "one size fits all" type of chart/comparison with this many variables.

## Telescope light gathering power and resolution - Astronomy

Largest refracting telescopes: 1-m primary lens.
Largest reflecting telescopes: 8-10 m primary mirror.

### LIGHT GATHERING POWER

• Number of photons collected proportional to exposure time multiplied by area of primary lens or mirror.
• Area = (pi / 4) x D 2 , where D = mirror/lens diameter.
• Diameter of primary determines light gathering power, hence ability to study faint objects.

### ANGULAR RESOLUTION AND SEEING

• Magnification depends on optics of secondary lenses or mirrors.
• More magnification = higher angular resolution, ability to see structure.
• Maximum resolution achievable is lambda / D radians, light wavelength divided by diameter of primary. Consequence of wave nature of light.
• Human eye (0.2-cm pupil): about 1 arc-minute.
• 10-cm (4-inch) telescope: about 1 arc-second.
• Achieving best resolution requires very accurate mirror surface, with correct shape to

## Improving resolution in large telescopes

New discoveries in astronomy require the use of ever larger telescopes. There are two motives for increasing the telescope aperture: greater light-gathering power and the potential for higher spatial resolution. The largest of these telescopes require segmented mirrors, since current technology does not provide for the fabrication of monolithic mirrors with diameters much larger than 8m. In addition, random errors in the positioning of the segments will give rise to random speckle effects in the astronomical images, which complicates the detection of faint structures. It would therefore be desirable to eliminate these adverse effects while maintaining the advantages of having a large aperture. 1

A possible solution for the static artifacts is to place a mask at the pupil plane of the telescope. There are many different proposals in the literature for masking. Some of them place a mask in each individual segment of the aperture. 2 Others are based on masking the full telescope aperture. Some approaches propose transmittance masks, whereas others prefer phase elements. In this work, we use circularly symmetric supergaussian amplitude masks in the full segmented aperture in order to improve image quality.

We chose the supergaussian profile for two reasons: it avoids sharp edges, thereby reducing diffraction effects, and it provides some degree of control over the shape of the point spread function (PSF), as shown in Figure 1. These supergaussian filters can be implemented dynamically using a spatial light modulator, or can be fabricated, for example, in High Energy Beam Sensitive (HEBS) glass (Canyon Materials), which reacts to electron-beam exposure. Varying the exposure allows the desired supergaussian profile to be written into the glass with very high resolution in position and intensity.

Unfortunately, telescopes never yield an ideal performance, owing to random piston and tip-tilt errors that affect the placement of each segment, gaps between the segments, distorted segment edges, and so on. Nonetheless, supergaussian filters can improve resolution and eliminate the static diffraction pattern due to the segmented telescope geometry even in the presence of moderate error sources. We have compared annular and supergaussian masks with different error sources: piston errors, tip-tilt errors, both piston and tip-tilt errors, and gaps between segments. We have checked that the supergaussian filter with exponent &alpha = 12 presents low sidelobes and the best Strehl ratio in every case, although different supergaussian filters may be useful in other circumstances. As an example, Figure 2 shows the comparison of several filters for the case of both piston and tip-tilt errors.

Furthermore, the majority of large telescopes are equipped with adaptive optics (AO) systems that adjust the mirrors to compensate for atmospheric distortions. However, it is impossible to obtain perfect adaptive compensation due to several error sources, including fitting errors and temporal delay between sensing and compensation. As a consequence, the compensated PSF consists of a coherent core surrounded by a speckled halo. The height of this residual halo depends on the turbulence strength and the AO system characteristics. It is therefore sufficient to reduce the diffraction pattern using supergaussian filters to a level below the halo that remains after partial correction by the AO system.

In summary, we have analyzed the use of supergaussian amplitude filters to decrease the structure pattern due to the segmentation geometry of large telescope mirrors. In addition, we have discussed how the characteristics of the AO system must be considered when choosing the filter. The future development of this technique faces several challenges. First, a thorough study of the instrument characteristics and limitations should be carried out (e.g., polishing and phasing errors, and random gaps between segments). Second, supergaussian filters need to be combined with complex filters that yield superresolution to attain the resolution predicted for larger telescopes. 3

This research was supported by the Ministerio de Ciencia y Tecnologa grant AYA2004-07773-CO2-01

To use the "information" telescopes collect in the form of light, one needs to have instruments that actually record that information. IMAGING (recording the "picture" of objects) was done on photographic plates. Now it is done on CCDs (Charged Coupled Devices). These are essentially electronic photographic plates whereby the amount of photons stimulates a reaction that produces charged particles. The information is imprinted and stored directly in the form of numbers which go into a computer and are then processed for analysis.

SPECTRAL ANALYSIS is the study of the shape of the spectrum coming from an object and to see this the light has to be separated according to its wavelength. We discussed this earlier in the context of a prism and a diffraction grating. Diffraction gratings are used to produce the spectra in sophisticated modern telescopes.

## 4 - Telescopes

The past decades have seen dramatic improvements in our observing capabilities. There have been improvements in our ability to detect visible radiation, and there have also been exciting extensions to other parts of the spectrum. These improved observing capabilities have had a major impact on astronomy and astrophysics. In this chapter we will first discuss the basic concepts behind optical observations. We will then discuss observations in other parts of the spectrum.

An optical telescope provides two important capabilities:

(1) It provides us with light-gathering power. This means that we can see fainter objects with a telescope than we can see with our naked eye.

(2) It provides us with angular resolution . This means that we can see greater detail with a telescope than without.

For ground-based optical telescopes, light-gathering power is usually the most important feature.

We can think of light from a star as a steady stream of photons striking the ground with a certain number of photons per unit area per second. If we look straight at a star, we will see only the photons that directly strike our eyes. If we can somehow collect photons over an area much larger than our eye, and concentrate them on the eye, then the eye will receive more photons per second than the unaided eye. A telescope provides us with a large collecting area to intercept as much of the beam of incoming photons as possible, and then has the optics to focus those photons on the eye, or a camera, or onto some detector.

## Telescope light gathering power and resolution - Astronomy

1. Some details .
• New office hours: Mondays 3:30-4:30 and Thursdays 2:00-3:00 PM in my office (DRL 2N3C).
• Homework #3 is now due on Monday 2/26/96 instead of today. Note that the web version has some clarifications and hints.
• Homework #4, handed out today, is due on Monday 3/4/96.
• The updated course schedule reflects the new weekly homework scheduling, and the new reading schedule.
• Next week we will cover Chapter 6 and hopefully start Chapter 7.
• I noticed on the midterm that many of you were confusing the factors of 2.5 and 2.512 that appear in the intensity - magnitude relation. Note that the 2.512 is actually 10^(1/2.5) or 2.5 = 1/log(2.512) approximately (the 2.512 is rounded). The 2.5 is exact, in the relations I_A/I_B = 10^( m_B - m_A / 2.5 ) or (m_B - m_A) = 2.5 log(I_A/I_B) (see below).
2. Images from Lenses and Mirrors
• A lens can be made from a refracting material such as glass that is shaped with oppositely curving surfaces such that parallel light rays passing through from one side are focused to a point, called the focus, on the other side of the lens.
• The distance from the center of the lens to the focus is naturally enough called the focal distance, which we will abbreviate as f.
• Note that the focus is on the line running through the center of the lens perpendicular to the plane that bisects the lens - this is called the optical axis. You can make a good lens out of two spherical surfaces, where the radius of curvature determines the strength of bending and thus the magnification. The useful lenses for our purpose are converging lenses, with convex surfaces so that the rays pass through and converge. If you make a lens with concave surfaces, the rays will diverge, which isnt too useful here!
• You can focus rays also with a curved mirror. It turns out that a parabolic mirror (shaped like a parabola) will focus incoming parallel rays also to a point along the optical axis. Note that the focus is of course in front of the mirror.
• The reason we consider parallel incoming rays is that we are most interested in looking a very far away objects like planets and stars. These objects are so far away that the light rays from them are essentially parallel by the time they reach the Earth.
• It is interesting to look at the image formation with a lens or mirror. Just by drawing an upright object, like an arrow, with the bottom on the optical axis some distance in front of the lens, then drawing the rays, we can see where an image is formed. You only need to know that rays parallel to the optical axis are bent to pass through the focus, and that rays through the center of the lens are not deflected.
• You find that the image is inverted (upside down!) and located some distance beyond the focus which depends upon the how far in front of the lens you drew the source, and how strong a lens you made it (what the focal distance was).
• There is a thin-lens equation that states: 1/f = 1/s_o + 1/s_i where s_o is the distance from the source object to the lens, and s_i is the distance from the lens to the image (and f is the focal distance).
• Note that if you move the object out to infinity in front of the lens, then the image moves closer to the focus. Thus, for astronomical purposes, the image of a distant star-field is made in the focal plane, the plane perpendicular to the optical axis and containing the focus.
• You can make the same exercise for the parabolic mirror. You also find an inverted image that moves closer to the focal plane as you move the object to infinity.
• In both cases for a single lens or mirror the image is inverted. Note that our eye is a single lens, and thus we see the world upside down (at least on our retina). Our brain has been trained from birth to ignore this and reprocess the information so that we perceive the world as right side up! Part of the fumbling about by little babies is learning to deal with inverted images in the eye. The optical processing by our brains is trainable - scientists have done experiments where special goggles that invert our view are worn by subjects, who of course are disoriented at first. After a few days, they find that they see the world not upside down but right side up again! This reverses when they take off the goggles, and their vision is returned to normal.
3. Refracting & Reflecting Telescopes
• You can make a simple refracting telescope by placing a large objective lens at the top of a tube, and then a second smaller lens called an eyepiece at the bottom end of the tube. The eyepiece is place just beyond the focus and its purpose is to focus the diverging rays into parallel rays again, which are then focused by our eyes just as if we were looking at the sky without the telescope.
• The focusing of the eyepiece controls the size of the image that appears on the eye, and thus the magnification of the telescope (see below).
• Note that a refracting telescope, or refractor is unwieldy to make, since you need a long tube for big lenses (since its hard to make large strongly curved lenses). If you've picked up a large magnifying glass, you also know big lenses are very heavy!
• You can make a reflecting telescope, or reflector, by placing a parabolic mirror at the bottom of a tube. The focus is then inside the tube, unless you put a second mirror in front of the focus to direct the light somewhere else.
• The large main parabolic mirror in a reflector is called the primary mirror, and its focus inside the tube is called the prime focus. Many large optical telescopes are big enough so that there is actually room inside the tube for a person, who can observe at the prime focus (with an eyepiece)! Many radio telescopes put their receivers at prime focus also.
• If a second mirror, or secondary mirror is placed in front of the prime focus, and is oriented at 45 degrees to the optical axis so as to reflect the light through a hole in the side of the tube, then we have what is called a Newtonian telescope, with a focus on the side and outside the telescope called the Newtonian focus. This is the first sort of reflecting telescope built 1668 by Isaac Newton (designed in 1663 by James Gregory).
• If you orient the secondary mirror perpendicular to the optical axis, then the converging rays are pointed back down toward the primary. If a small hole is made in the center of the primary, then the focus can be place at the bottom end of the telescope where it is very convenient for observing. This design is known as a Cassegrain telescope, and the focus is of course the cassegrain focus.
• With extra mirrors, there are an number of different configurations where the focus is directed to convenient places outside the tube. See the textbook for some of these.
• Note that both for refracting and reflecting telescopes, the image quality degrades (becomes distorted and blurry) as you look at stars that are at increasing angles from the optical axis. This is because the simple lens or parabolic mirror only has a point-like focus for rays parallel to the optical axis - rays at slight angles actually focus into a little blurry spot that gets bigger the further you go off-axis. Thus the useable field-of-view, the angle from the center of the image that is not terribly distorted, is fairly small for these telescopes.
• For a refracting telescope, complicated lenses with different surfaces and extra lenses can be used to "correct" for these aberrations. For a reflector, the secondary mirror can be specially figured and a large thin correcting plate or lens can be placed at the top of the tube. If this is done to a Cassegrain telescope, then it becomes a Schmidt-Cassegrain telescope (the lens is called the Schmidt corrector). This is the most popular sort of serious astronomical telescope.
• Note that if a Schmidt corrector is place on a prime-focus telescope, you get a telescope with an excellent wide field of view. If you place a photographic plate or other detector at the prime focus, then you get a Schmidt Camera. These are used to make sky surveys, with a single photographic plate that covers as much as 6 degrees!
4. The Powers of a Telescope
• Three "powers" of a telescope:
1. Light Gathering => brightness of images
2. Resolving => sharpness of images
3. Magnifying => size of images
• There are a good number of powerful telescopes now operating around the world. Below is a quick survey of some of the best of these. You can also explore the internet on your own by using your favorite Web searching engine using "telescope" or "observatory" as a keyword (try this).
• Although most optical telescopes operate alone as a single (often large) telescope, some radio telescopes are linked together to form what are called interferometers, or interferometer arrays.
• An interferometer array is like one large telescope made up of smaller telescopes - it has the resolution of a single telescope of a diameter equal to the largest distance between two telescopes in the array, but has the light gathering power of the sum of the collecting areas of the individual telescopes (usually much less than the area of a single telescope the size of the array). Thus, an interferometer can give superb resolution for even modest size telescopes.
• It is difficult to link an interferometer together. You need to adjust things to a fraction of a wavelength. This is easiest in the radio part of the spectrum, where wavelengths range from 1 mm to 1 meter. In the optical, with wavelengths of 1000 nm or less, this is extremely difficult as you might imagine! There are several groups of astronomers around the world working on this important technological problem.
• For more on interferometers, see the next lecture.

Focusing in a single thin lens:

Image formation with a single thin lens, and the thin-lens equation:

Focusing with a single paraboloidal mirror:

Image formation with a paraboloidal mirror:

A simple compound refracting telescope, with objective and eyepiece:

Three types of reflecting telescopes: prime focus, Newtonian, and Cassegrain:

The Schmidt Camera and the Schmidt-Cassegrain Telescope:

The Light Gathering Power and image brightness:

Resolving Power and resolution of telescope:

The ESO Very Large Telescope (VLT) is now under construction in Chile.

The twin 8-meter telescopes are being built the US National Optical Astronomy Observatory (NOAO) on Mauna Kea, Hawaii and in Chile.

For many years, the 200-inch (5-meter) Hale Telescope at Palomar Observatory in California was the largest telescope in the world. It was finished in 1948. Here is a detailed construction drawing by Russell Porter from 1938:

The Green Bank Telescope (GBT) is a large steerable radiotelescope being constructed in Green Bank, West Virginia, by the National Radio Astronomy Observatory (NRAO).

The NRAO Very Large Array (VLA) is an interferometric array of 27 antennas, each 25-meters in diameter, placed on railroad tracks on a high plain near Soccorro, New Mexico. This interferometer make radio images with the resolution of a single telescope 36 km across!

The Very Long Baseline Array (VLBA), constructed by NRAO, consists of nine 25-meter telescopes

located at sites across the USA from Hawaii to the Virgin Islands.

The longest baseline is 8612 kilometers, and the resolution is as good as a telescope of this enormous size!

The soon-to-be-launched VLBI Space Observatory Program (VSOP), is a 8-meter antenna on a Japanese satellite that will orbit the earth out to distances of 2.6 Earth diameters. In conjunction with ground-based VLBA telescopes, this will give a baseline of over 33000 km!

The NRAO has proposed building an interferometric Millimeter Array (MMA) that will do what the VLA does but at shorter wavelengths of 0.3 to 3 millimeters. The current design is for 40 8-meter telescopes arranged on a circular track in the high mountains of northern Chile.

## Light Gathering Power of Binoculars.

Say one is using 100mm binoculars, is the total light gathering power of the instrument based on just one of the objectives, or is the total light gathering based on both objectives combined?

In other words, do 100mm binos have the light gathering power of a 8" scope, or just a 4"?

May sound dumb, and it probably is, but I can't figure it out in my head! Since both eyes are used, I can see it going either way.

### #2 Jim Nelson

This isn't just an optics question it's a visual neuroscience question.

However, it certainly wouldn't be the equivalent of an 8" scope an 8 inch scope has 4x the light gathering power (surface area) of a 4inch scope, not 2x.

### #4 half meter

140% -- that's a number I've heard also. So your 100mm
binos would act like a 140mm telescope in terms of
effective brightness.

Resolution, though, would be better in the 140mm scope.

### #5 Richard McDonald

40% is about right. Let's do the math:

100mm diameter objectives are radius 50mm.
Area = pi * r^2
Area = 7853 mm^2

Two such objectives, is twice the area, 15707 mm^2

if a = pi * r^2 then r = sqrt(a/pi)
so 15707 mm^2 is equivalent to r=71mm
or diameter of 142mm

So, two 100mm objectives is equivalent to a 142mm objective, about 40% more.

### #6 mnpd

Thanks fellows. So a 4" inch optic has pi x (2x2) = 12.5" of surface area, and an 8" has pi x (4x4) = 50" of surface area.

A 100mm (4") binocular will have the 12.5" of a normal 4" optic plus the extra 40% which comes out to be approximately 24". So it's not as efficient as the combined total of both 100mm objectives, but considerably better than just a 100mm objective by itself.

Because of the "visual neuroscience" issue it's a bit hard to quantify. But, you got me on the right track.

### #7 Glassthrower

Vendor - Galactic Stone & Ironworks

Two 100mm objectives = bright images.

They're great for faint fuzzies.

### #9 Jim Nelson

This thread has a post by Ed Zarenski where he estimates that the *effective light gain* is from 20-40%, NOT the increase in effective aperture. If that's the case, then a pair of 100mm binoculars are equivalent to 110mm to 120mm telescope.

I'll poke around a bit more and see if I can get more specific information.

Lots more discussion and info here:

Hmmm. I'm having trouble repairing that link just cut and paste the line into your browser address line.

### #10 Jon Isaacs

>>>Here's a question that keeps crossing my mind.

Say one is using 100mm binoculars, is the total light gathering power of the instrument based on just one of the objectives, or is the total light gathering based on both objectives combined?

In other words, do 100mm binos have the light gathering power of a 8" scope, or just a 4"?
----

As others have pointed out, the combined light gathering power is not that of a 8 inch scope but rather of a 4 in x sqrt (2) = 5.66 inch scope.

The binocular view will give you better performance than a 4 inch scope but a 5.66 inch scope used with one eye will not only provide better resolution but allow one to go deeper in terms of faint targets.

### #12 Jon Isaacs

As i say again ,overall light of 100mm bino is like 141mm
objective.But this does not make a difference for eye .
100 mm bino wil be like 100mm objective,only benefit
is stereo vision - who could argue this.
-----

The issue is really how effectively the brain processes and combines the images from each eye. By closing one eye when using binoculars, it is quite clear that one sees more with both eyes so there is some real advantage here. It is my understanding however that one eye and a single 141mm scope will show more.

### #13 snorkler

I agree with Kruno. There is no increase in light gathering power from using binoculars over a monocular. Those of you claiming this need only go to a dark room, look at a dim object, and move a hand in front of one eye. Did the object dim by 20% or 40%? No, it didn't.

I'm sure we may perceive more with two eyes, just like we can hear more with two ears. If I sit in front of stereophonic speakers and move a hand in front of one ear about a foot in front of that ear, I'll lose the high notes coming toward that ear, but my overall loudness level doesn't decrease.

The binocular vision effect probably varies depending on the person. I'd expect a right-eye dominant person or a left-eye dominant person to perceive different light losses/gains v. a person with no eye dominance.

### #14 half meter

What we see is based on a signal-to-noise ratio. Using two
eyes helps boost this ratio so we "see" more.

It's the same principle as stacking many 5 second CCD images
to get the equivalent of a 30 second image. How could that
work if it weren't for increasing the signal-to-noise ratio?

### #15 kruno

So we agree that light gathering stays same,but stereo vision
improves picture (only on binos-binoview must have twice light gathering to compensate separation).

I only wonder if brain has capability to use interferometric capability,like we use, when connecting two separate telescopes to get more resolving power.

### #17 Jon Isaacs

>>>I agree with Kruno. There is no increase in light gathering power from using binoculars over a monocular. Those of you claiming this need only go to a dark room, look at a dim object, and move a hand in front of one eye. Did the object dim by 20% or 40%? No, it didn't.
-----

I suggest the proper experiment is to take those binoculars out under the night sky, let your eyes adapt and then compare difference that closing one eye makes.

When I do this, I find that the difference between one eye and two is definitely noticeable.

Note that binoculars really do capture twice as much light, they just spread it out between your two eyes. How the brain processes the imformation, well, that is a complex matter but the simple experiment outlined above shows that one can see more.

### #19 Jon Isaacs

See the link here for an explanation of the phenomenon. Again, it's not from binoculars gathering more light. It's the way our brains process light coming from two eyes v. one.
=========

It is a number of factors. Two eyes with 70mm binos vs 1 eye with a 100mm telescope, the scope will show more. (Same objective area.)

### #20 kruno

that observer with binos will EASIER percept more detail but will not see deeper in magnitude than observer with
monocular with same objective area and same magnificatio.
But this is objectionable too.
Experienced observer with monos will see almost as much (even can say same) detail as observer with binos.

Object on Earth will make much more difference,they are not in infinity so stereo vision makes 3d difference..closes the
object more difference its make.So bino observer will see whole lot more detail but in dimension monocular can not show at all.

Only difference on infinite objects can be interferometric
but this is questionable and will be small not detectable difference,except you have bino with big separations between
objectives. few meters example.

so my conclusion is that stargazers with binos have more pleasant and easier view and thats all.

### #21 mnpd

Why are You doing this,as You well know each half of bino gathering for each eye so 100mm will be equivalent to
100mm.Other thing is that stereo vision is more pleasant
and relaxing and eye could go "deeper".
It would be great to have some kind of adapter which will combine light from both parts of bino into one -only then will You have 15000 sq-and also interfered resolution of 200mm scope ----but for one eye only .
Am i missing something ?
Maybe You only discussing of overall light gathering ?
k

Yes, the question is only about total light gathering capability. It's an academic question as opposed to an aesthetic one.

### #22 EdZ

The light gathering of a 100mm binocular is not based on the total area of the two binocular lenses. It is, as some have mentioned above, based on binocular summation. For light gathering and contrast, binocular summmation provides 120% to 140% the benefit of a single aperture. BUT that is based on area of aperture, not diameter of aperture.

So the correct answer is 100x100 = 10,000 x 1.4 = 14000. Sqrt 140000 = 118mm.
A 100mm binocular provides (at best) the equivalent contrast and light gathering of a 118mm single aperture.

Lot's of other things do change, for instance as Jon points out, resolution would not be equivalent. The 118mm scope will have better resolution.

### #23 kruno

i do not agree with this and stays on mine previous statement.
only benefit will be in eye and brain biology.
"BUT that is based on area of aperture, not diameter of aperture.100x100":question:
to me 100mm bino will be :
(2*100mm bino/2eyes) x eye&brain factor = aprox.100mm
I do not know much about biology but from article
somebody posted link before seems to me is near around 1
for light,slight great than 1 for resolution..

Try to see deeper star mag with 100mm bino than in 118 mm scope,
i dont think so.

-sorry for this of topic-
But if You manage to merge light from both objectives
(mirrors) but for one eyepiece only then You will get
light gathering equivalent 140mm objective.
And resolving power objective = distance between binos objective axes + objective diameter (greater than 200mm - depends on bino).
But nobody using this -it will be to complicated and small difference for such a small aperture.And sight will not be pleasant but scientific. real small interferometer(theoreticly).

### #24 EdZ

Please refer to the post pinned at the top of the bincular forum titled

Links to Web - Binocular Info.

In that post you will find links to various web articles from some of the most well known and renouned authors who have written on the subject of astronomy, binoculars and vision. I would suggest reading all of the articles found through those links. There is some very useful information that will help you gain a better understanding of vision, optics and binocular summation, just to highlight a few topics. Start out selectively reading the info on Summation and Vision. Delve into some of the other topics as time permits.

I have actually performed tests using binoculars and telescopes of various sizes and have proven the studies for binocular summation give an accurate value for light gathering. The results of those tests have been published here on these forums for several years. You may find those through the Binocular "Best Of" links under the thread titled "Two Eyes versus One."

Binocular Summation IS a neural process. But it cannot be ignored. The amount of information procesed by two eyes is significantly greater than that processed by one eye.

Those of you above that are coming upwith the answer 141mm are taking a mis-step in the calculation. A 141mm lens has (nominal) 20,000 sq mm area. That would be the result if you were able to deliver the light from two 100mm lenses to ONE eye. That is not wht happens when you have light from 100mm lens to each eye. Again, back to binocular summation.

Many years of binocular vision testing has proven that binocular vision produces gains over one eyed vision. Those gains were pointed out above. And you can search out the story if you wish, but this has been proven. You need only follow through with an understanding of limiting magnitudes and LM gain.

To gain a full magnitude, you would need to increase the area of aperture by 250%. In order to measure these gains and compare scopes to binculars, you need to take many readings on precise star magnitudes. A gain f 4/10 magnitude is a 100% gain in light. A gain of about 40% in light is approximately a gain of 2/10 magnitude.

In order to perform these tests of binocular vision, you must test scopes and binoculars at the same magnification. Sure I can take any type of 118mm scope and use it at about 100x or 120x and I would be able to reach a LM of about mag 13 or slightly deeper. But take that 118mm scope and use it at 25x, the same power as a 25x100 binocular. You will find that you are near equal to the 25x00 binocular. In the case of my best skies about mag 5.7, that is near or just greater than mag 12.

If you do all of the above, you will have proven one other very important aspect of light gathering. And that is, you cannot see all the light an instrument gathers unless you use optimum magnification. You can also find an explanation of this in the links provided above. For LM, optimum magnification is that which provides an exit pupil of about 1mm. That is not what you have in a 25x100 binocular. So you must test the scope at the fixed power of the binocular. When you do this, it proves the science behind binocular summation. And yes, it will vary by individual. That is why the range is stated 20% to 40% gain.

Binocular Suummation will provide a gain of about 20% to 40% over that provided by a single aperture to one eye. For me it is closer to 40% and for light gathering that is about a 0.2 magnitude gain over a scope of same single aperture at the same magnification. Contrast follows the same rule, so it is easier to see extended objects in a binocular compared to a scope of same aperture same power.

There will be very slight gains in resolution, but very slight and difficult to measure by percent. As was pointed out above, binocular summation cancels out interference. When the signal is delivered by two eyes, the brain is more readily able to reduce interference in the image and the gain is real. But for resolution it is not 20% or 40%.

## Mountain-Top Observatories

• Dark Skies, far from large cities
• Clear, Dry Weather
• Good "seeing" (steady atmosphere to reduce twinkling & smearing of images)
• The Chilean Andes near the Atacama Desert
• The summit of Mauna Kea on Hawaii
• Mountains in Arizona (Kitt Peak, Mt. Hopkins, Mt. Graham)

To see what the Earth looks like at night from composite satellite imagery (the source of the image of the US at night I showed in lecture), visit the Images page of the International Dark-Sky Association.

## Telescope light gathering power and resolution - Astronomy

An overview of the current state of submillimeter and infrared astronomy is given. In order to develop these fields, three areas must be considered. First, a platform immuned to atmospheric effects must be found, and satellites capable of supporting large telescopes must be designed. Current programs are considering specialized instruments such as COBE, a small cosmic background explorer IRAS, a small cooled infrared survey telescope and SIRTF, a small cooled infrared telescope. Second, a large area telescope with light gathering power and resolution, comparable to that available in the optical and radio, is essential to the program. Recent NASA studies have indicated the feasibility of constructing a 20 m diameter telescope with a 20 micron wavelength diffraction. Third, detectors are being developed which are near quantum noise limited, radio-style detectors. Questions which can be answered by submillimeter and infrared techniques pertain to star formation, existence of other planetary systems, and missing mass formation.